I have and extra cool fuel assembly off of a 500efi if that would work

Have you tried a new MEFI or IC module? Maybe somebody like whipple or Arizona Speed have MEFI 1s traded in.

Just my opinion, but I wouldn't go through all the trouble and expense to remove the EFI and convert to a carburetor until I had exhausted ALL the options to troubleshoot and repair the original problem.

Sometimes it's something as little as a corroded ground or a pin in the main connector that's not making proper contact.

what he said

Sure, I hear you; I've been trying all summer to get the EFI to work properly.
I've been working with Mark at Precision Marine. He has been vary helpful remapping ECUs and such.
I've got the 3rd MEFI-1 ECU on it now and I've replaced lots of other stuff too.
Yes, I've check all wiring 3 times and gone thru many diagnostic procedures.
It's just this old MEFI-1 stuff is out of date and only available used.
I may convert to a MEFI-3 system next year, but that is even more $$ than going to a Carb...
BOATS...

I have a carb and intake.
I will need to buy an electric fuel pump.

You guys are telling me I can not use the distributor, right?
I think others have used them, but...

Now, what to do with the engine wire harness?

You never told us the symptoms. What is it doing wrong? No start, rough idle, no power, hot start problems? You might just detail something that one of us has run into before.

I would bet just between MobileMercMan, Raylar, Gellner and myself we have thousands of hours of diagnostic time put in. It is part of our "dues" and the Lord knows we've paid them!

how can it be a mefi 1 with the coolfuel set up ? my understanding was that all the cool fuel set ups were the late ones w/ the mefi 3 ...

I've had a couple of other threads that you guys have chimed in on and been vary helpful. I thank you all for that.
My story is a long one but basically the original ECU failed in a continuous knock mode and had it running rich and the timing retarded. The transom still shows signs of that.
I bought another ECU from a 1994 502MagMPI and the engine ran OK and without fault codes. The idle was not great though and after consulting with you guys I sent the ECU to Mark for reprogramming. That did not work out so great and Mark is checking to see if that ECU has now failed.
Mark loaned me another ECU to try and now I have new fault codes (coolant temp sender). I when through the diagnostics and there is 5vdc at the pins so I'll change the sender and see what happens, but I've about had enough with this 1996 system. I'm not buying another MEFI-1 computer that is for sure.
The engine seems to be OK mechanically; compression test was 150-155 for all. I have not done a leak down yet.
The engine does not run right above 3900rpm, it stumbles and seems like it's not getting fuel or the spark is cutting out.
Fuel pressure is OK.

My new thread is to see how easily and how much $$ to convert to my carb and intake that I have sitting on the self…

I do have a MEFI-3 computer and harness from a 310HP/7.4MPI, but like I said early, not this year…

1996Merc 502MagMPI has a coolfuel and MEFI-1 computer.

Pat,

I am very interested in this project, as I'd love to go the other direction on my engine. After years of working with EFI both at GM as well as on my own, I'm completely in awe at just how HORRIBLE a job a carb does even when it's tuned "perfectly". Cyl-cyl fuel balance is horrible, resulting in having to run the entire engine way richer than it should need. If you keep the manifold cool for max power, it kills idle and fuel economy. If you size the carb right for part throttle, you lose 20hp on top....etc, etc. All compromises I NEVER had to deal with on EFI.

I wish I was near you. I'd love to be able to help you out. I want to get some experience with MEFI. One possible solution is to keep the complete fuel system, but change to an aftermarket ECU.

If you do go with the carb, I'll be interested in how much driveability you think you lose.